Several methods are used for tracking and providing information about items. For example, inventory items in stores typically bear printed labels providing information such as serial numbers, price, and size. Some such labels include printed symbols, such as bar code symbols which may be selected from a variety of symbologies, so that they may be read optically. Such labels contain a very limited amount of information and are not easily updated, typically requiring the label to be reprinted if it is to be modified.
An alternative method of tracking and providing information about devices uses parasitic power memories. Parasitic power memories are commercially available devices having a single input line and a low power memory device, typically used as tags for inventory control and identification. An example of such devices is found in the DS199x series of Touch Memory devices available from Dallas Semiconductor Corporation.
In operation, parasitic power memories are usually read using a special electronic reader that physically contacts the parasitic power memory and forms an electrical connection to the parasitic power memory. The reader supplies a 5 V signal to power the parasitic power memory and outputs a coded signal (polling code) to access the parasitic power memory. When the parasitic power memory receives the polling code and the power input, it outputs data serially to the reader or accepts data from the reader. A further discussion of the structure and operation of such devices is found in the Book of DS199x Touch Memory Standards and the release 50 Ways to Touch Memory, each by Dallas Semiconductor and each of which is incorporated herein by reference.
Such memories disadvantagnously require the reader to physically contact the memory to provide power, input the polling code and output the data. Also, the reader must be programmed with a priori knowledge of the particular parasitic power memory, such as control information, including the proper polling code and data transfer protocol. There is no simple mechanism for the reader to obtain control information from the memory itself.